Plating Q&A: Alternatives to Hydrochloric Acid
Coventya’s Brad Durkin explain how to reduce your first pass defect rate.
Q. We barrel and rack plate zinc from acid zinc, alkaline zinc and zinc nickel electrolytes on machined components manufactured from 1215 free cutting, resulfurized and rephosphorized steels and sometimes on 12L14 leaded steel substrates. We often experience a high percentage of blistering of these deposits after plating, which requires rework increasing our production cost, reduced throughput and higher waste treatment cost. We use a hydrochloric acid pickle diluted to 40 to 45 percent volume with an inhibitor which does not overcome these defects. We have been told to add fluoride to our acid pickle process, but adding the fluoride as hydrofluoric acid (HF) to our existing plating activation process won’t be permitted by our company safety authority, and we need to reduce our first pass defect rate. What input can you provide?
A. The types of substrates described are chosen for parts that require machined surfaces where the additions of lead and similar species to the substrate alloy will improve the machining processing. Unfortunately for the plating applicator, these can be detrimental, increasing the potential for plating defects.
Hydrochloric acid (HCL) and its mixtures are traditionally used on the majority of zinc process lines because of the success in providing good activation for processing most carbon steel substrates. More importantly, HCL is good for stripping zinc deposits from plating danglers, racks and other associated equipment, which is critically important in a zinc plating production facility. Traditional HCL concentrations for zinc production plating lines range from 25 to 35 percent volume dilutions of concentrated 37 percent weight HCL. Higher concentrations, especially those that require the use of inhibitors to minimize surface attack, can create problems with resulting tenacious surface films formed on the substrate surfaces. Plating over these films will easily occur, but that weakness in the bonding of the zinc to the substrate can cause immediate or delayed blistering. Also, drag-in of these inhibitors to acts as contamination that can cause other problems with plating bath performance. If possible, it’s best to avoid the use of inhibitors on plating production lines unless specified by the part manufacturer for minimizing the potential for hydrogen embrittlement.
Fluorides added to HCL pickles on zinc plating lines have been successful in removing surface smuts from resulfurized and rephosphorized subtrates, especially for residual concentrations of lead that are smeared on part surfaces from the machining process. These residues, if not properly removed, will inhibit good adhesion of zinc plating to part surfaces.
The benefits of fluoride ions in activating many substrate types are well documented throughout many sources in our industry and enable successful plating and metallization in production environments for a host of substrates and conditions. For example, ASTM standard guide B319 (2004) for electroplating leaded alloys states that an acid pickle is necessary for a fluoride-containing acid to remove all oxides and insoluble lead compounds that remain on the surface as a result of surface preparation. Steel alloys of 12L14 (leaded), and in the case of the alloy 1215, where there is some indication of lead impurity, the benefit of using fluoride should not be underestimated. From a safety perspective, HF concentrates should be avoided because it is known to be aggressive, corrosive, more difficult to use, store and transport and can potentially cause severe burns when handling.
In the plating industry, ammonium bifluoride (ABF), a dry salt, is a good replacement for HF. In general, bifluorides have many uses and applications and are safely handled in many types of industries including: glass cleaning, monument and building cleaning, oil well acidifiers, wood preservatives, disinfectants in water chemistry and fluorination of water.
True, the addition of ABF salt to the HCL pickle will liberate free fluoride ions. Dissociation of the salt is inevitable, though desirable for the benefit and functionality with much less safety risk. For example, it is common practice in other plating applications to use salts of acid, such as sodium nitrate salt in acid mixtures, to receive the benefit of the nitrate anion activity while avoiding the handling of concentrated nitric acid solution.
The target for introducing 15 to 30 grams per liter of ABF to the existing HCL acid activation/pickle is to facilitate dissolving the surface smut and aiding in the removal of the oxides, while lessening the negative impact from lead residues that typically remain on the surfaces from these alloy steels. Verifying that associated equipment is compatible with fluorides in the line is also an important consideration. This addition would also reduce the HCL concentration to a more traditional 30 to 35 percent volume per volume concentration, which is consistent with many zinc plating finishing operations on the market today. Reducing or eliminating the requirement for any introduction of inhibitors to the acid pickle in this process is also important.
Successful quality plating requires a good balance, beginning with the conditions of substrates coming into process lines with corresponding output success. A properly designed plating process will clean oils, remove surface smuts and oxides while providing the proper activity of the surface for accepting the plating.
Brad Durkin is the director of international product management for Coventya. For more information, please visit coventya.com.
Originally published in the May 2016 issue.
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